Many electronic devices use for their operation one or several periodic logic signals, for example to synchronize a phase-locked loop or to be used as clock signals. For different reasons, the periodic input signal provided to a device may disappear and cause a malfunction of the device. Further, in some cases, when for example a CRT control device receives a periodic horizontal synchronization signal to generate control signals for the tube deviator, the disappearing of the horizontal synchronization signal can result in the generation of inappropriate control signals and the destruction of the deviator.
When such periodic signals are compulsory for some devices, a conventional solution consists of very clearly specifying in the technical instructions of the devices which signals must imperatively be provided during operation to avoid damage.
Although it is always possible to use this solution, it is preferable to detect the disappearing of the periodic logic input signal that can cause a serious malfunction and, as soon as this detection is achieved, to modify the device operation to avoid any risk of damage.
Further, it is very important to be able to perform this detection very fast, to limit damage that may occur between the signal disappearing and the detection of this disappearing.